Three-dimensional rotation angiography (3D rotation angiography) is a standard method used for estimating the vascular anatomy before and during interventions. The reconstruction algorithms used for 3D reconstruction of the cardiac blood vessels by means of rotation angiography are heavily dependent on the periodicity of the cardiac motion. In the interventional environment patients often have arrhythmic heart signals or cannot hold their breath during a complete acquisition.
For diagnostic examination purposes and for interventional procedures in for example cardiology, radiology and neurosurgery, interventional X-ray systems are used for imaging, the typical essential features of which systems can be a C-arm on which an X-ray tube and an X-ray detector are mounted, a patient positioning table, a high-voltage generator for generating the tube voltage, a system control unit, and an imaging system including at least one monitor. The C-arm can be held for example by means of a robot arm. A C-arm X-ray system of this kind, as illustrated for example in FIG. 1, has a C-arm 2 which is rotatably mounted on a stand in the form of a six-axis industrial or articulated-arm robot 1 and at the ends of which are mounted an X-ray radiation source, for example an X-ray tube assembly 3 with X-ray tube and collimator, and an X-ray image detector 4 as image acquisition unit.
The articulated-arm robot 1 known for example from U.S. Pat. No. 7,500,784 B2, which preferably has six axes of rotation and hence six degrees of freedom, enables the C-arm 2 to be moved to an arbitrary position in space, for example by being rotated around a center of rotation between the X-ray tube assembly 3 and the X-ray detector 4. The inventive X-ray system 1 to 4 can be rotated in particular around centers of rotation and axes of rotation in the C-arm plane of the X-ray image detector 4, preferably around the center point of the X-ray image detector 4 and around axes of rotation intersecting the center point of the X-ray image detector 4.
The known articulated-arm robot 1 has a base frame which is permanently installed on a floor for example. Attached thereto is a carousel which is rotatable about a first axis of rotation. Mounted on the carousel so as to be pivotable about a second axis of rotation is a robot rocker arm to which is attached a robot arm which is rotatable about a third axis of rotation. Mounted at the end of the robot aim is a robot hand which is rotatable about a fourth axis of rotation. The robot hand has a retaining element for the C-arm 2, said retaining element being pivotable about a fifth axis of rotation and rotatable about a sixth axis of rotation running perpendicular thereto.
The X-ray diagnostic apparatus is not dependent on the industrial robot for its implementation. Conventional C-arm devices can also be used.
The X-ray image detector 4 can be a rectangular or square, flat semiconductor detector which is preferably produced from amorphous silicon (a-Si). Integrating and possibly counting CMOS detectors can also be used, however.
A patient 6 to be examined is placed as the examination subject in the beam path of the X-ray tube assembly 3 on a patient positioning table 5 so that images of the heart, for example, can be recorded. Connected to the X-ray diagnostic apparatus is a system control unit 7 having an imaging system 8 which receives and processes the image signals from the X-ray image detector 4 (control elements are not shown, for example). The X-ray images can then be viewed on a monitor 9.
Body electrodes 10 which are placed for example on the thorax of the patient 6 can record the ECG signals of the patient 6 and transmit them to the system control unit 7.
A respiration sensor 11 for recording the respiratory motion of the patient 6 lying on the patient positioning table 5, which sensor can be for example a chest belt placed around the thorax of the patient 6, measures the respiratory motion of the patient 6 and transmits it to the system control unit 7.
The X-ray tube assembly 3 emits a bundle of rays 12 originating from a beam focus of its X-ray radiation source and striking the X-ray image detector 4. If it is intended to generate 3D data sets according to the so-called DynaCT method for low-contrast visualization of for example soft tissue, as described for example in US 2006-0120507 A1, the rotatably mounted C-arm 2 with X-ray tube assembly 3 and X-ray image detector 4 is rotated in such a way that, as FIG. 2 shows schematically in a view onto the axis of rotation, the X-ray tube assembly 3 represented figuratively here by its beam focus as well as the X-ray image detector 4 move around an examination subject 13 located in the beam path of the X-ray tube assembly 3 on an orbit 14. The orbit 14 can be traversed completely or partly for the purpose of generating a 3D data set.
In this case the C-arm 2 with X-ray tube assembly 3 and X-ray image detector 4 moves according to the DynaCT method preferably through an angular range of at least 180°, for example 180° plus fan angle, and records projection images in rapid succession from different projections. The reconstruction can be carried out based on just a subset of said acquired data.
The subject 13 to be examined can be for example an animal or human body or indeed a phantom body.
The X-ray tube assembly 3 and the X-ray image detector 4 each rotate about the object 5 in such a way that the X-ray tube assembly 3 and the X-ray image detector 4 are disposed on opposite sides of the subject 13.
In normal radiography or fluoroscopy by means of an X-ray diagnostic apparatus of this type the medical 2D data of the X-ray image detector 4 is buffered in the imaging system 8 if necessary and subsequently displayed on the monitor 9.
By applying the methods of computed tomography (CT) it is aimed to generate a 3D image of the coronary vessels from the 2D projection data of a moving heart. For this purpose it is important to take into account the movement of the heart. Particularly in the case of the long recording times of the C-arm systems of at least 5 s other movements can also take place in addition to the beating motion of the heart. While periodic motions (heartbeat) can be frozen by means of ECG gating, non-periodic motions such as for example arrhythmic heartbeat, respiration or patient movement must be estimated and corrected in the reconstruction.
With ECG gating, a specific cardiac phase can be singled out from the recording data. The recording time of conventional CT scanners is currently in the region of about 100-200 ms, such that within a single heartbeat all of the measurement data required for reconstructing a 3D image for a specific cardiac phase can be recorded. Since the recording time of C-arm systems is about 5 s, measurement data of a plurality of cardiac cycles must be used for reconstructing a 3D image for a specific cardiac phase, as is described for example in Schäfer et al [1]. An assumption of ECG gating is the periodicity of the motion. With long recording times the periodicity is no longer guaranteed due to arrhythmic heartbeat, respiration, patient motion.
Furthermore gaps in the recording data are produced as a result of ECG gating. There are approaches aimed at using all the measurement data of an image acquisition by correcting the motion of the heart in the reconstruction step. Estimating the motion from the recorded image data represents a challenge. Motion estimation is a very poorly formulated problem. The degrees of freedom of a general motion are very great and cannot be clearly determined from the measured recording data.
In the publication by C. Blondel et al [3] the motion estimation problem is limited based on the assumption that the motion is periodic, although this is not the case in reality.
E. Hansis et al [5], Rohkohl et al [6] and H. Scherl et al [7] also take non-periodic motions into account in their estimation. The complexity of motion estimation is limited through use of an a priori 3D image. A search is made for a motion in order to reconstruct a 3D image which is very similar to the a priori 3D image. Typically the a priori 3D image is generated from a reconstruction using ECG gating. With non-periodic motion, the image quality of the a priori 3D image and consequently also the image quality of the main reconstruction suffer.
U.S. Pat. No. 7,415,093 B2 describes a CT cardiac diagnostic imaging method which uses a priori information from motions from 3D ultrasound examinations using ECG gating. ECG and ultrasound data of the heart is acquired in real-time during a scan. A data acquisition module is controlled during the scan so as to predictively control the acquisition of the CT data as a function of the real-time ECG data and the real-time ultrasound data. A 3D image is reconstructed from the acquired CT data. The motion field is estimated from the ultrasound examinations and used as a priori knowledge.
DE 10 2004 048 209 B3 discloses a precise and comparatively easy-to-implement method for generating a three-dimensional image data set of a moving object by means of X-ray tomography as well as a device that is particularly suitable for performing the method and has a rotatably mounted X-ray tube assembly detector unit as well as an evaluation unit, wherein it is provided to group a number of two-dimensional raw images according to a cyclical relative time, to generate at least two preliminary 3D image data sets in each case from raw images corresponding to one another according to said grouping, to derive at least one motion matrix by comparison in each case of two preliminary 3D image data sets used as source data set and target data set, to generate a motion-compensated 3D image data set corresponding to a reference time of the source data set through application of the or each motion matrix onto the associated target data set, and to sum the or each motion-compensated 3D image data set with at least one further motion-compensated 3D image data set or another preliminary 3D image data set corresponding to the same reference unit. The motion field is therefore estimated from first preliminary 3D images from different cardiac phases (time instants). The method has the disadvantage that if there is considerable movement no usable first preliminary 3D images can be generated.
U.S. Pat. No. 6,070,097 A describes a method for generating a gating signal for cardiac examinations by means of an MRI system comprising a detector system which receives an ECG signal from a scanned patient and generates a gating signal when a detected peak in the ECG signal meets a set of R-wave criteria which include a specified positive slope on the leading segment of the detected peak, a minimum duration of the leading segment, a specified negative slope on the segment trailing the detected peak and a minimum peak amplitude. An ECG signal is therefore analyzed in order to detect the QRS pulse at which ECG gating is to take place.